Silicone vitrimers prepared by vulcanisation of pendant vinylpolysiloxanes with elemental sulfur

Abstract

In this study, pendant vinyl functionalised polysiloxane oils were converted into dynamic, self healing elastomers after crosslinking with elemental sulfur through vulcanisation. Three polysiloxanes with varying vinyl content, chain length, and functional groups were directly reacted with elemental sulfur (S₈), generating three different crosslinked materials: X-poly(PDMS-PVMS-r-S), X-poly(PVMS-r-S), and X-poly(PPMS-PPVS-r-S), where X is wt% sulfur. The reactions were monitored by solution state ¹H NMR spectroscopy, confirming progressive vinyl consumption by the decrease in vinyl proton signals (H = 5.7–6.2 ppm), and simultaneous appearance of CH/CH2–S bonds (H = 2.0–3.0 ppm). Vitrification times varied across the three polymers, primarily influenced by the polysiloxane chain length, vinyl content, other pendant functional groups, and sulfur loading, with higher sulfur content generally leading to shorter times. Solid state NMR spectroscopy on the final cured elastomers confirmed vinyl consumption and CH/CH2–S bond formation, and thermogravimetric analysis (TGA) showed a lower char yield with higher sulfur incorporation, consistent with the increased proportion of thermally labile polysulfide linkages. Glass transition temperatures measured via DSC were higher in the phenyl containing polysiloxane, up to –3°C, compared to the methyl rich polysiloxanes (Tg < –70°C) due to the reduced backbone mobility from the bulky phenyl substituents. Contact angle measurements confirmed that all elastomers remained highly hydrophobic (104–111°). Rheological analysis demonstrated increasing tan δ values with sulfur content, attributed to a higher density of dynamic crosslinks. After physically damaging the materials and thermal healing, efficient S–S bond reformation restored mechanical integrity, while minor irreversible changes modestly influenced viscoelasticity. In addition, DMF enabled a solvent-based route to depolymerisation–recrosslinking. 10-poly(PPMS-PPVS-r-S) dissolved fully in DMF, whereas 10-poly(PVMS-r-S) and 7-poly(PDMS-PVMS-r-S) were only partially soluble. After removing DMF and briefly annealing (140°C, 1 h) the elastomers were regenerated, providing chemical recyclability alongside thermal healing. These results show that pendant-vinyl polysiloxanes can be directly converted into dynamic, repairable elastomers through catalyst and solvent-free vulcanisation using elemental sulfur, a refinery by-product, as the sole crosslinker. The process is 100% atom economical and generates materials which extend silicone lifetimes while valorising industrial sulfur waste.